1. Field of the Invention
The present invention relates to an oxidation apparatus for forming an oxide film on an object to be processed such as a semiconductor wafer (to be referred to as a wafer hereinafter).
2. Description of the Related Art
A conventional oxide furnace is of a horizontal type. In such a horizontal furnace, a boat on which 150 wafers for example are mounted must be loaded in/unloaded from a process tube by a cantilever fork. However, a deposit is adhered to the inner wall surface of the furnace and the boat by the last annealing step. Therefore, if the fork flexes by its own weight, the deposit adhered to the boat is separated. As a result, the deposit is adhered to the wafers as dust or an impurity.
In addition, the horizontal oxidation furnace requires a large installation space. However, the cost per unit area in a clean room is high. Therefore, a strong demand has arisen for space saving of an oxidation furnace especially for wafers. However, the conventional horizontal oxidation furnaces cannot satisfy this requirement.
As for the above mentioned problems, it is difficult to automatically control the horizontal furnace or to increase its diameter. In addition, in the horizontal furnace, a temperature difference is produced between spaces above and below a wafer due to convection in the furnace, resulting in a poor yield. Furthermore, the intake of external O.sub.2 is large. As a result, in the horizontal furnace, an unnecessary oxide film is formed and therefore the film thickness on a wafer is difficult to control.
Conventionally, in order to form an oxide film on the surface of a wafer, a wet oxidation method is adopted to obtain a steam atmosphere in a reaction tube.
As the wet oxidation method, a method called pyrogenic oxidation is generally utilized. In this method, as shown in FIG. 1, hydrogen gas 1 and oxygen gas 2 are burned and combined in a combustion chamber to generate steam. This steam is guided to a reaction tube 4 which houses wafers W, thereby obtaining a steam atmosphere in the tube 4.
Note that in FIG. 1, reference numeral 5 denotes a wafer boat; 6, a gas burner; 7, an oxygen supply path; 8, an oxygen and hydrogen flame; and 9, a heater for heating hydrogen gas up to its ignition temperature. Then, high-temperature oxygen and hydrogen flame 8 comes closer to inlet port upper wall portion 10 of combustion chamber 3 to cover portion 10. Since chamber 3 is formed of quartz, it is denatured when it is heated to a high temperature. In the worst case, chamber 3 is melted and broken. According to a conventional steam generation method, hydrogen gas and oxygen gas is combusted and combined in the vicinity of the wall of a combustion chamber made of quartz, in order to prevent an accident explosion due to non-combustion of hydrogen gas.
As a result, the wall of the combustion chamber is heated and the transparency of the wall is lost. In addition, molecules in the wall of quartz, i.e. so called impurities, are dispersed in the combustion chamber and adhere to steam.
If such steam is employed in the oxidizing process or diffusing process of semiconductor devices, the resultant semiconductor devices have low quality. Furthermore, since a tip portion of a hydrogen nozzle must be positioned near a heat source, it is not possible to freely set the location at which hydrogen gas is combusted.